The present invention relates to a composite magnetic member combining a ferromagnetic portion and a non-magnetic portion in a single material, which member can be used in industrial products utilizing a magnetic circuit, such as a motor.
Industrial products requiring a magnetic circuit, such as the rotor of a motor and a magnetic scale etc., conventionally have a structure in which a non-magnetic portion is provided in a part of a ferromagnetic body (generally, a soft magnetic material). Techniques such as the brazing and laser welding of a ferromagnetic part and a non-magnetic part have been employed to provide a non-magnetic portion in a part of the ferromagnetic part. In contrast to these techniques of bonding dissimilar materials, the present inventors propose the use of a single material as the material for a composite magnetic member which is formed by providing a ferromagnetic portion and a non-magnetic portion by cold working or heat treatment. When such composite magnetic members made of a single material are used, it is possible to obtain parts superior to those obtained by bonding a ferromagnetic portion and a non-magnetic portion regarding the respects of ensuring airtightness, ensuring reliability, such as prevention of breakage by vibrations, etc., and reducing the cost thereof.
In JP-A-9-157802 based on the proposal by the present inventors, for example, a martensitic stainless steel containing 0.5 to 4.0% Ni is disclosed as a composite magnetic member suitable for an oil controlling device of an automobile. This proposal is such that in a martensitic stainless steel composed of ferrite and carbides in an annealed condition, by adding Ni of an appropriate amount in a Fe--Cr--C base alloy in which such a ferromagnetic characteristic as to be not less than 200 in maximum magnetic permeability, a non-magnetic portion having magnetic permeability not more than 2 is obtained and is stabilized in the martensitic stainless steel through the steps of heating the portion and then cooling it, and that the Ms point (at which the austenite begin to be changed to martensite) can be lowered to a temperature not more than -30.degree. C.
Also, JP-A-9-228004 based on another proposal by the present applicant discloses that, by adding more than 2% but not more than 7% Mn and 0.01 to 0.05% N to a C--Cr--Fe-base alloy containing 10 to 16% Cr and 0.35 to 0.75% C and having ferromagnetic properties with a maximum magnetic permeability of not less than 200, there is obtained a composite magnetic material used in magnetic scales, etc., in which material a retained austenite with a magnetic permeability of not more than 2 is obtained and is stabilized by cooling after heating, and it becomes possible to lower the Ms point to not more than -10.degree. C. These proposals are excellent in the respect that a ferromagnetic portion with a maximum magnetic permeability of not less than 200 and a stable non-magnetic portion with a magnetic permeability of not more than 2 and a low Ms point can be obtained in a single material.
The composite magnetic members disclosed in the above JP-A-9-157802 and JP-A-9-228004 are based on the proposal that a non-magnetic portion stable down to low temperatures can be formed in a part of a ferromagnetic body by adding an appropriate amount of Ni and Mn, which are austenite-forming elements, to a martensitic stainless steel from which ferromagnetic properties can be obtained, and by performing partial solution treatment, and it can be said that these composite magnetic members are excellent in the respect that a single material can combine a ferromagnetic portion with a maximum magnetic permeability (.mu.m) of not less than 200 and a stable non-magnetic portion with a magnetic permeability (.mu.) of not more than 2.
According to examinations by the present inventors, some of the composite magnetic members used as a magnetic circuit are required to have better soft magnetic properties (hereinafter referred to as soft magnetism) than those of conventional members, i.e., high maximum magnetic permeability and low coercive force, for example, as in the rotor of a motor. In contrast to this, in the above two proposals there were limits to the soft magnetism obtained in the ferromagnetic portion.